Subchondral bone injury in racehorses

Racehorses repeatedly subject the subchondral bone of the fetlock and carpus to extreme loads, which results in material fatigue and accumulation of microdamage. The relationship between load and fatigue life of subchondral bone (number of cycles of load before failure) is exponential. Higher speed generates higher load, so higher speeds are disproportionally associated with subchondral injury. Therefore, with every fast gallop, microdamage in the subchondral bone accumulates. However, subchondral bone can adapt to training and racing loads by modelling, as well as repair damaged bone through bone remodelling. Adaptation occurs through increased bone volume, which increases resistance to fatigue. Where the rate of microdamage accumulation is greater than the rate of damage repair, larger cracks form in the superficial subchondral bone, which propagate transversely. A focal repair response may be successful or may exacerbate the problem by undermining the articular surface resulting in its collapse into the underlying bone. Management of subchondral bone injury involves treatment of associated pain and inflammation, and time out of intense training to allow for appropriate healing. Most horses can return to racing successfully provided the condition is not too advanced, but there is usually some reduction in performance.